PROJECT SUMMARY/ABSTRACT: revised Helicobacter pylori causes clinical disease primarily in those individuals infected with a strain that carries the cytotoxin associated gene pathogenicity island (cagPAI). The cagPAI encodes a type IV secretion system (T4SS) that is required for injection of the CagA oncoprotein into epithelial cells and induction of the proinflammatory cytokine, interleukin-8 (IL-8). CagY is an essential component of the H. pylori T4SS that has an unusual sequence structure, in which an extraordinary number of direct DNA repeats is predicted to cause rearrangements that invariably predict in-frame insertions or deletions. We have demonstrated in murine and non-human primate models that immune-driven host selection of rearrangements in CagY is sufficient to cause gain or loss of function in the H. pylori T4SS. We hypothesize that CagY functions as a sort of molecular rheostat that alters the function of the T4SS and ?tunes? the host inflammatory response so as to maximize persistent infection. We propose three specific aims to test this hypothesis. In Aim 1 we will determine the mechanism by which recombination in CagY alters the function of the H. pylori T4SS. Isogenic strains expressing variant CagY proteins that confer a functional or non-functional T4SS will be used to determine the capacity of CagY to bind and signal through ?1 integrins, characterize the topology of CagY in the bacterial membrane, examine the interaction between CagY and other essential PAI proteins, and determine if the DNA repeats are essential for T4SS function. In Aim 2 we will characterize the role of host immunity in selection for CagY-mediated modulation of function in the H. pylori T4SS using knockout and transgenic mouse models. The goal of Aim 3 is to better understand the physiological role of CagY and T4SS function using the highly relevant rhesus macaque model and strains from chronically infected patients. Completion of these experiments will characterize a novel strategy by which a bacterial secretion system alters the host immune response, and identify the mechanisms where the T4SS and host immunity intersect. These experiments will also enhance our understanding of the relationship between the PAI and the clinical outcome of infection, and lead to a broader understanding of the relationship between chronic infection and inflammation.